The shear descent isn't really a return - it's a water feature. Returns return water to the pool at a level below the water surface.
The PV3 IFCS uses a single MDX-R3 main FLOOR drain (they call it a debris drain) and the system also requires secondary suction sidewall drain. The single sidewall drain is connected to the floor drain to be VGB compliant.
The 2 sidewall drains that are closest together (there is a minimum distance required) are connected and feed the second pump (the one without the filter).
The IFCS will be adversely affected by going to a smaller impeller or you would need to run the variable speed pump at a higher rpm to attain the necessary flow rate to adequately clean the pool. Running at a higher rpm with a smaller impeller would keep the pressure the same as it is now - defeating the purpose of using a smaller impeller in the first place.
To alleviate the SWG experiencing pressure fluctuations, it should be plumbed to the second pump. The SWG chlorinated water can be sent back to the pool via the single return or via the shear descent - thru the existing 3 way valve on that return line. This would require the least plumbing changes. There are other options, but they would require more extensive plumbing changes and the possible addition of a second (and appropriately sized - normal sized) filter - in addition to some plumbing changes to separate the SWG and IFCS. The IFCS main drain and the water valve need to be plumbed thru the same pump.
Hopefully you will receive a direction from the manufacturer and your PB will step up and fix it all free of charge.